Why do my particles grow outside the density vertex group?
One vertex group.



The other vertex group.

If you have multiple vertex group make sure you are viewing and editing the correct one.

Also make sure your blue color is exactly blue.  Paint it again to be sure.  Zoom in from different rotation angles for nooks and cranies.  Rotate the model so you are always facing the mesh face directly while painting.  Delete the vertex group, create it again, and use the new name in the Particle System.